IR and Raman spectroscopy reveal amino acid–surface interactions on B- and N-doped hydroxylated graphene quantum dots: a DFT study

Abstract

Graphene quantum dots (GQDs), available in a variety of sizes and morphologies, have emerged as versatile nanomaterials with broad applicability across numerous fields, particularly in biomedicine. Most experimental and theoretical studies have focused on either dopant effects or surface functionalization independently, often using relatively large graphene models. A molecular-level understanding of how B/N doping and hydroxyl functionalization jointly influence biomolecular adsorption and spectroscopic signatures at the ultrasmall GQD scale remains limited, motivating the present DFT investigation of amino acid–GQD interactions. In this study, we investigate the physisorption behavior of individual amino acid molecules on pristine and hydroxyl-functionalized GQDs, as well as on their B/N doped counterparts, to gain insights into potential interactions relevant to protein environments. All calculations were performed using density functional theory (DFT) at the M06-2X/6-31G(d,p) level. Pristine GQDs with a lateral dimension of 1.3 nm, together with their singly/doubly doped and hydroxyl-functionalized variants, were fully optimized. Electronic properties and vibrational signatures were obtained through IR and Raman spectral analyses. Glycine (Gly) and serine (Ser) were subsequently adsorbed onto the modified GQD surfaces to quantify adsorption energies and assess changes in their electronic and spectroscopic properties. For hydroxyl-functionalized GQDs, the most stable adsorption configurations involved the formation of dual hydrogen bonds between the functional groups and the amino acids. The relative positioning of dopant atoms significantly influenced the stabilization or disruption of π-electron density across the GQD surface. These structural modifications produced notable enhancements in electronic properties, including band-gap modulation and increased affinity for noncovalent interactions. Overall, both functionalization and doping substantially improved amino acid adsorption, regardless of amino acid type.